Background: Heat stress in the steel industry is considered as a serious threat to the health and safety of workers. A wide spectrum of complications and diseases from mild disorders such as burning sensation to fatal conditions such as thermal shock may occur due to uncontrolled thermal stress. In this study, heat stress assessment was performed according to the wet-bulb globe temperature (WBGT) index among workers of a steel mill in 2014.Materials and Methods: In this cross-sectional study, 11 stations of a steel mill were selected in which occupational exposure to heat stress was possible. The WBGT index was used to measure heat stress. The collected data were analyzed using descriptive statistics in SPSS software.Results: The results showed that the highest exposure to heat stress was in the rolled operator unit (31.5 °C) and the least exposure to heat stress was in the main operator room (18.52 °C). Personnel working in the poker furnace operator, lift operator, ruffing operator, wrench operator, rolling work operator, scissors operator, and lathe operator units were exposed to higher heat stress than the threshold limit value for heat exposure recommended by the American Conference of Governmental Industrial Hygienists (ACGIH).Conclusions: The findings of this study showed that steel mill workers are exposed to heat stress and the heat conservation planning intervention action should be conducted to reduce exposure.